Polyester-carbonate resins are especially desirable for use in producing molded and extruded articles, particulaly, for glazing and transparent sheet applications demanding high impact resistance, high resistance to scratching, and/or high solvent resistance. They exhibit melt processability, high tensile strength, high impact strength and good clarity and freedom from color; approximating like qualities in commercial polycarbonates. Further, they are for some uses even more desirable than polycarbonates due to their higher heat distortion temperature. However, the usefulness of polyester-carbonates is limited by their high susceptibility to yellowing during exposure to light, especially sunlight.
During accelerated light aging studies, unstabilized polyester-carbonate is noticeably yellow (change in yellowness index of about 5) after less than 5 hours of exposure to RS sunlamps. By way of comparison, BPA-polycarbonate, which is considered to prone to photo-yellowing, must be exposed to RS sunlamps for about 100 hours to become noticeably yellow.
It is, therefore, an object of this invention to provide polyester-carbonate articles and a method of producing such articles which are highly resistant to ultraviolet light degradation.
The use of ultraviolet radiation absorbers with various resins such as polyesters, polyolefins, vinyls, polycarbonates and polystyrene to provide protection against attack by ultraviolet radiation is known in the art. The ultraviolet radiation absorber functions by reason of its ability to screen out the damaging ultraviolet portion of light due to its very high absorptivity relative to that of the polymer. In order to qualify as a successful ultraviolet light absorber for a polymer, particularly for polyester-carbonate, there are several requirements which the absorber must fulfill. The absorber must have a high specific absorptivity in the range of wave lengths that are most deleterious to the polymer and that are present in the source of the exposure. The absorber must be compatible with the polymer and/or coating composition and must not degrade either the polymer or coating composition so as to cause a loss of properties and increase in color. The absorber must not significantly absorb in the visible region of the spectrum or a color will be imparted to the polymer to which it has been added. The absorber must also have a sufficiently low volatility to permit its continued residence in the polymer.
Additionally, because the UV absorber is applied to the polyester-carbonate article in a coating composition, it is important that the coating composition is durably and tenaciously adhered to the surface of the article and is compatible therewith so as not to adversely affect the underlying polyester-carbonate as by stress cracking or crazing, by causing crack propagations and/or adversely effecting the properties of the polyester-carbonate generally, as for example impact resistance, elongation and tensile strength.